1. Field of the Invention
The present invention relates to apparatuses for reproducing sound by headphones or speakers with the sound image(s) being located at any position(s) outside the head of a listener or around the listener.
2. Description of the Related Art
In recent years, multi-channel audio signals have been used frequently for sound which accompanies video such as movies, and are recorded on the assumption that the sound is reproduced by speakers disposed at both sides and the center of a screen or a display where the video is displayed, and by speakers disposed after or both sides of the listeners. With this, the sound source in the video matches the sound image from which the sound apparently comes, and a sound field having a normal range is obtained.
When such sound is reproduced by headphones, however, the sound image produced by an input audio signal is located in the head of the listener, the video position does not match the sound-image locating position, the sound image is located at a position extremely strange, and the sound-image locating position of an each-channel audio signal cannot be independently separated.
Even when only multi-channel sound such as music is listened to, if the sound is reproduced by headphones, unlike a case in which the sound is reproduced by speakers, the reproduced sound image is located in the head of the listener, the sound-image locating positions of the multi-channel audio signal are not separated, and a sound field extremely strange is obtained.
Therefore, in a case in which sound is reproduced by headphones, an idea has been examined in which the sound images are located at any potions outside the head of the listener to provide the same sound field as that obtained when speakers are disposed at those positions.
FIG. 22 shows the principle of the idea in a case in which two-channel stereo sound is reproduced by headphones with the sound images thereof being located at any positions outside the head of the listener, for example, at right-hand and left-hand positions symmetrical against the center plane before the listener.
In this case, transfer functions (frequency responses) HRR and HRL from a sound source 5R where the sound image is located to the right and left ears 1R and 1L of the listener 1, and transfer functions HLR and HLL from a sound source 5L where the sound image is located to the right and left ears 1R and 1L of the listener 1 are obtained in advance by calculation or by measurement in which right-hand and left-hand speakers are disposed at the positions of the sound sources 5R and 5L and right-hand and left-hand sound output therefrom is measured at the positions of the right and left ears 1R and 1L of the listener 1.
FIG. 24 shows a conventional audio reproducing apparatus used for the case shown in FIG. 22. Right-hand-side and left-hand-side analog audio signals Ar and Al corresponding to the signals of the sound sources 5R and 5L shown in FIG. 22 are input to terminals 11R and 11L, and are converted to digital audio signals Dr and Dl by A/D converters 12R and 12L, the digital audio signal Dr is sent to digital filters 21RR and 21RL, and the digital audio signal Dl is sent to digital filters 21LR and 21LL.
The digital filters 21RR and 21RL convolute impulse responses to which the transfer functions HRR and HRL are converted in a time domain, into the digital audio signal Dr. The digital filters 21LR and 21LL convolute impulse responses to which the transfer functions HLR and HLL are converted in a time domain, into the digital audio signal Dl.
An adder circuit 22R adds the output signals DRR and DLR of the digital filters 21RR and 21LR. An adder circuit 22L adds the output signals DRL and DLL of the digital filters 21RL and 21LL. The output digital audio signals DR and DL of the adder circuits 22R and 22L are converted to analog audio signals by D/A converters 13R and 13L. The two-path analog audio signals are amplified by audio amplifier circuits 14R and 14L, and sent to the right-hand and left-hand acoustic transducers 3R and 3L of headphones 3.
Therefore, in the audio reproducing apparatus shown in FIG. 24, the transfer functions HRR and HRL are demonstrated through the paths of the digital filters 21RR and 21RL, and the transfer functions HLR and HLL are demonstrated through the paths of the digital filters 21LR and 21LL to locate the sound images of the right-hand and left-hand input audio signals Dr and Dl at the positions of the sound sources 5R and 5L.
When sound is reproduced by speakers, a speaker layout is usually restricted. A limited number of listeners can place a great number of speakers for reproducing multi-channel sound in their listening rooms.
Therefore, an idea has been examined in which a great number of sound images produced by multi-channel input audio signals are located at any positions around the listener by a small number of speakers, for example, by two speakers.
FIG. 23 shows the principle of the idea in a case in which speakers 6R and 6L are disposed at right-hand-side and left-hand-side positions symmetrical against the center plane before the listener and the sound image of an input audio signal SO is located at any position around the listener, for example, at a left-hand rear position indicated by a sound source 7.
In this case, the relationships between the input audio signal SO, which is the signal of the sound source 7, and driving signals SR and SL for the speakers 6R and 6L are expressed as follows:SL=HL×SO  (1)SR=HR×SO  (2)
HR and HL indicate transfer functions expressed by the terms to be multiplied by the signal SO in expressions (1) and (2), and are functions of transfer functions HRR and HRL from the speaker 6R to the right and left ears 1R and 1L of the listener 1, transfer functions HLR and HLL from the speaker 6L to the right and left ears 1R and 1L of the listener 1, and transfer functions HOR and HOL from the sound source 7 to the right and left ears 1R and 1L of the listener 1, with cancellation of a cross talk between the speakers 6R and 6L being taken into account. The transfer functions HRR, HRL, HLR, HLL, HOR, and HOL are measured or calculated in advance.
FIG. 25 shows a conventional audio reproducing apparatus used for the case shown in FIG. 23. An analog audio signal Ai is input to a terminal 11, and is converted to a digital audio signal Di by an A/D converter 12, the digital audio signal Di is sent to digital filters 21R and 21L.
The digital filters 21R and 21L convolute impulse responses to which the transfer functions HR and HL are converted in a time domain, into the digital audio signal Di.
The output digital audio signals DHR and DHL of the digital filters 21R and 21L are converted to analog audio signals by D/A converters 13R and 13L. The two-path analog audio signals are amplified by audio amplifier circuits 14R and 14L, and sent to the speakers 6R and 6L.
Therefore, in the audio reproducing apparatus shown in FIG. 25, the transfer functions HR and HL are demonstrated through the paths of the digital filters 21R and 21L to locate the sound image of the input audio signal SO (Di) at the position of the sound source 7.
FIG. 25 shows a case in which the sound image of a one-channel audio signal is located at one sound-source position. When a sound-image-locating signal processing section formed of the two digital filters 21R and 21L shown in FIG. 25 is provided for each of multi-channel audio signals, a great number of sound images produced by the multi-channel audio signals can be located at any positions around the listener by the two speakers 6R and 6L.
In the conventional audio reproducing apparatuses shown in FIG. 24 and FIG. 25, the digital filters 21RR, 21RL, 21LR, 21LL, and 21R and 21L convolute impulse responses, such as that shown in FIG. 2, to which the transfer functions HRR, HRL, HLR, HLL, and HR and HL are converted in the time domain, respectively, and are formed of a finite-impulse-response (FIR) filter such as that shown in FIG. 3.
In this case, more specifically, the input audio signal Di (Dr or Dl) is sequentially delayed by delay circuits 51 connected in multiple stages, each having a delay time of the sampling period (τ) of the input audio signal. Each multiplier circuit 52 multiplies the input audio signal Di (Dr or Dl) or the output signal of each delay circuit 51 by a coefficient corresponding to the impulse response thereof at each sampling period τ. Each adder circuit 53 sequentially adds the output signal of each multiplier circuit 52 to obtain the output audio signal DHR (DRR or DRL) or DHL (DLR or DLL) after filtering.
The digital filters 21RR and 21RL, 21LR and 21LL, or 21R and 21L may be formed, as shown in FIG. 4, of a structure in which delay circuits 51 are shared, a multiplier circuit 52 and an adder circuit 53 form one digital filter, and a multiplier circuit 54 and an adder circuit 55 form the other digital filter.
In this case, however, if the impulse response such as that shown in FIG. 2 is not sufficiently extended in time for an input audio signal for each channel, reproducibility deteriorates especially at low frequencies of several hundred Hz and lower, and a clear feeling of sound-image locating is not obtained at the low frequencies.
When the numbers of orders (taps) of impulse-response-convolution digital filters are increased, for example, when the number of stages of the delay circuits 51 in an FIR filter, such as that shown in FIG. 3 or FIG. 4, is increased, the impulse response is extended in time.
Then, however, when the sound-image-locating signal processing section is formed of hardware, the circuit scale becomes huge, and when the sound-image-locating signal processing section is formed of hardware and software (program) like a digital signal processor (DSP), a huge amount of calculation is required.